1. Field of the Invention
This invention relates generally to sorting machines and particularly to an illumination source for a machine vision system.
2. Description of the Related Art
Sorting machines incorporating machine vision systems typically identify and sort articles by means of reflected energy waves. One of the main components of a vision machine system is the illumination source. The illumination source provides a starting point for the reception quality of the vision system. Typically, the source is required to be uniform and have a high intensity at the object point (sometimes referred to as the scan line) of the vision system. Most inspection systems include some sort of light source. Conventional light sources include incandescent and fluorescent lamps and light emitting diodes. Various optical arrangements have been designed for better illumination, such as ringed lamp arrays, focused filament projectors, and fiber optic emitters. Uneven illumination in conventional illuminators may result in detection of shadows as defects. While the characteristics commonly measured incorporate light sources including human-visible light sources, machine vision systems may measure energy waves outside the human-visible range.
U.S. Pat. No. 6,355,897 to Bjork describes an arrangement and method for sorting granules that includes a light detector arranged over a transparent pellet transportation track and a light source arranged on the opposite side of the track. The detector is at one end of a chamber with the light source and track at the other end. The chamber is evenly illuminated and may have a reflective layer. The light source may also illuminate the pellets from above or around the track. Defects are indicated as a lower intensity potential at the detector.
U.S. Pat. No. 5,201,576 to Squyres discloses a spherical chamber, which is covered with a reflective interior surface, with a light source within the chamber. A transparent tube extends through an axis of the chamber. The objects to be inspected are transported through the tube. At least two viewing openings are provided in the chamber with inspection cameras oriented through the viewing openings. The patent discloses the use of an acrylic white paint manufactured by Krylon, and claims that product's capacity to provide reflectivity above 90%. The patent further discloses use of titanium oxide coating as being prior art in optical integrating spheres.
The chamber is provided with a circularly tubular lamp, two video cameras and a transparent, cylindrical tube having two open ends. The objects are conveyed through the tube, illuminated by the lamp and examined by the cameras. One problem that may occur in connection with this solution is it may be difficult to adjust the cameras without affecting the light distribution inside the chamber. This is due to the fact that the intensity from the lamp, which is described in the U.S. Pat. No. 5,201,576, will vary inside the chamber, due to the fact that the intensity is higher close to the lamp than at a certain distance from the lamp. Another problem may be that the tube affects the light refraction in the form of reflections, e.g., that a mirror image of the lens may appear. Additionally, this solution is limited to inspecting serial objects, one side at the time.
U.S. Pat. No. 6,238,060 to Bourn et al. describes a ring light source of light emission diodes or similar points of light for providing focused, uniform light without shadows on a spot where an object may be inspected. The patent shows many variations; however, none is believed appropriate for a long scan line.
U.S. Pat. No. 6,234,317 to Sommer describes a number of light sources each within a light-transmissive cylinder that can be wiped or pneumatically cleaned from time to time. The objects pass between the light-transmissive cylinders during the inspection process.
U.S. Pat. No. 5,745,176 to Lebens describes a source having a linear array of lights and a focusing element intermediate the light source and the object to be viewed for producing a focused light on the object. The source has a background that prevents internal reflections that would otherwise interfere with the focused light and produce variations in light intensity from the source. The object that is inspected is not a moving object.
U.S. Pat. No. 5,586,663 to Graudejus, et al. describes a rotating background that can be kept clean. However, it is not a cylinder that surrounds the path of the inspected objects.
It would be an improvement to the prior art to provide an illumination system for a machine vision system that provides intense, even illumination of the articles to be viewed along a linear or elongated scan line, thereby providing consistent identification of selected characteristics and substantially reducing mis-characterization of articles as having occlusions or other defects genuinely caused by shadows.